If we want to compute the nth Fibonacci number we just power the matrix
$M =
\left[\begin{array}{cc}
1 & 1 \\
1 & 0
\end{array}\right]$
$n$ times and we get $M =\left[
\begin{array}{cc}
F_{n+1} & F_n \\
F_n & F_{n-1}
\end{array}\right]$
But, How should be the elements arranged when I want to find the $n$th term of the fibonnaci series whose starting elements are $a$ and $b$?